Bacteria aren’t the only microorganisms that find the human body an inviting place to live. Fungi forms of which exist in nature as mushrooms and in your bathtub as mildew can be found in abundance on the body’s surfaces, including the skin, mouth, and nasal passages.

Bacteria and fungi share an important characteristic: most of the time they’re unobtrusive visitors, but in the right circumstances, they can multiply, prompting discomfort and illness.

On the skin, an overgrowth of certain fungi causes everyday annoyances, such as athlete’s foot and jock itch. Fungi’s role in the nasal passages is a bit more complicated. We know that fungi can and do reside in both the nose and sinuses. But how often do fungi cause sinusitis?

That question has been the subject of great debate among researchers in recent years. A group of scientists at the Mayo Clinic challenged conventional wisdom that bacterial infections are the underlying cause of sinusitis and proposed that fungi were the true culprits in most cases.

I’ll cover this “fungal theory” in a bit, but first let’s look at some scenarios where there’s little doubt that fungi are directly related to the sinusitis.

Types of Fungal Sinusitis

There are three types of infections in which unusually large numbers of fungi are present in the sinuses:

fungus ball sinusitis

allergic fungal sinusitis

invasive fungal sinusitis

These three diagnoses account for less than 5 percent of all sinusitis cases.

Fungus Ball Sinusitis

The fungi that normally live harmlessly inside your sinuses occasionally begin to multiply. Like a bacterial sinusitis, this process may be triggered by blockage of the sinus ostia. But instead of bacteria overgrowing, for some reason it’s the fungi.

Growth may be slow or rapid, but if it continues unchecked, the sinus eventually fills up with a ball of thick fungal debris. Much like a bacterial infection, this process can cause facial pain and pressure. Fungus ball sinusitis is typically diagnosed by CT scan.

Whereas a bacterial infection often shows fluid inside the sinus (due to the presence of pus), the fungus ball tends to be more solid in appearance. Also, unlike most forms of sinusitis, a fungus ball usually causes an infection in a single sinus, typically one of the maxillary sinuses.

Medications usually aren’t effective against fungus ball sinusitis, so often the only option is a surgical procedure in which the fungus ball a gelatinous, green mass is scooped out of the sinus with a tool known as a curette.

Following surgery, sinus function quickly returns to normal, and most patients need no further treatment. Individuals with fungus ball sinusitis fit into the category of Locals on the Sinusitis Spectrum.

Allergic Fungal Sinusitis

Similar to pollen or dust allergies, people may have a reaction to the fungi living in their nose and sinuses, a condition known as allergic fungal sinusitis (AFS). The allergic reaction may cause the mucous membrane lining the sinuses to become inflamed.

In addition, very thick mucus, called allergic mucin, collects within the sinus. This mucin is typically dark green in color, with the consistency of peanut butter. Many people with AFS also have polyps.

The combination of mucin and polyps often blocks the sinus ostia, causing a secondary bacterial infection. A CT scan usually reveals thick mucus and debris filling sinuses on both sides of the head.

This debris is composed of white blood cells called eosinophils, as well as stringy filaments called hyphae, which are a form of the fungus. Because of the presence of eosinophils, AFS is also known as eosinophilic fungal rhinosinusitis.

Treatment usually starts with steroid sprays to shrink polyps and antibiotics to control any concurrent bacterial infection. In some cases, oral steroids successfully relieve symptoms.

In most cases, however, surgery is eventually needed to clear the thick debris within the sinuses and remove any polyps. Although surgery has a high likelihood of improving symptoms for the short term, in many patients, the underlying allergic mechanism persists and infections return, requiring repeat procedures.

On the Sinusitis Spectrum, those with AFS are considered Systemics.

Invasive Fungal Sinusitis

The final type of fungal sinusitis is rare but potentially fatal. It occurs only in patients whose immune systems are severely compromised. Examples include individuals with HIV or AIDS; people who have uncontrolled diabetes; those receiving chemotherapy for cancer; and organ transplant recipients who are taking immunosuppressant drugs.

Because these patients’ natural defense mechanisms are weakened, fungi are able to attack the sinus walls and invade the underlying blood vessels and bone. Uncontrolled infection can extend to the eyes, causing blindness, and to the brain, causing meningitis.

Invasive fungal sinusitis is diagnosed by a CT scan that shows destruction of the bony sinus walls and by examination that shows nonliving tissue in the nose of a severely ill patient.

If a case is recognized early enough, radical surgery to remove the infected tissue in combination with intravenous antifungal agents can be lifesaving.

A Larger Role for Fungi?

Some researchers believe fungi’s role in sinusitis goes beyond the three types I’ve described. In fact, they think fungi cause almost all cases of chronic sinusitis.

This novel viewpoint made headlines in 1999, when researchers at the Mayo Clinic published a study showing fungi are present in mucus samples from nearly all patients with sinusitis.

Until then, other researchers had mostly focused on tissue samples and pus (which contains bacteria) to diagnose sinusitis, but not mucus (which contains fungus). The Minnesota researchers theorized that fungi trigger an influx of eosinophils in certain patients prone to sinus infections.

These eosinophils attack the fungus by releasing inflammationinducing substances, such as major basic protein (MBP), the presence of which irritates the sinus lining and puts patients at risk for a secondary bacterial infection.

Treatment that focused on eliminating fungi, they said, would help huge numbers of patients whose sinusitis never seemed to go away. A follow-up study from the Mayo Clinic suggested that people’s symptoms improved when they irrigated their nose with topical antifungal agents.

Because of this, many ENT doctors began treating their most difficult chronic sinusitis patients with nasal irrigations and nebulized solutions containing Amphotericin B (amphocin). Others placed these hard-to-treat patients on weeks or months of the oral antifungal medication itraconazole (Sporanox).

Despite the initial enthusiasm, subsequent results have been unimpressive. Many observers now attribute the early improvements seen in some patients to the cleansing effects of nasal irrigations rather than the antifungal agent.

No controlled studies have emerged demonstrating the benefits of antifungal agents for the treatment of chronic sinusitis. It is also noteworthy that the 1999 Mayo Clinic report showed the same fungi that were growing in sinusitis patients were also growing in the mucus of a control group of subjects who did not have sinusitis.

Perhaps the fungi reside peacefully in all of our sinuses, just as they do in our mouths and on our skin, causing only occasional infections in select circumstances.

For now, the fungal theory of sinusitis remains controversial. I’m among the skeptics. I do believe eosinophils play an important role in the development of sinusitis, but I doubt that the presence of fungi is the “missing link” that explains why most people get sinusitis.